Pulsed Air-jet Actuators for Flow Separation Control

The design and validation of a microfabricated pulsed air-jet actuator for practical application to flow-separation control at full-scale operating conditions on a medium/large air vehicle is presented. The actuator device is designed to generate streamwise vortices within the boundary layer and comprises a pitched and skewed orifice of 200 μm diameter through which a high velocity (200–300 m/s) jet of air can be modulated by operation of a piezoelectric microvalve. This paper describes the overall design and manufacture of the actuator device with particular reference to the impact of fluid dynamic effects on the design and operation. Key results obtained from both static and dynamic tests of a prototype device are also presented and compared with original predictions. It is shown that the device that was developed and tested fulfils all the original design requirements with regard to size, jet velocity and operating frequency. The developed device has dimensions of approximately 5 mm × 2 mm in the plane of the aerodynamic surface in which it is imbedded and a thickness of 1 mm. Peak jet velocities in excess of 300 m/s through a 200 μm diameter orifice at 500 Hz have been demonstrated with peak driving voltages of 90 V and a nominal electrical power consumption of 50 mW.     

Modified flapping jet for increased jet spreading using synthetic jets

The present paper is an experimental investigation, using a PIV system, on modified rectangular jet flow co-flowing with a pair of synthetic jets placed symmetrically with respect to the geometric centerline of the main flow. The objective was to determine the optimal forcing conditions that would result in jet spreading beyond what would be obtained in a simple flapped jet. The main jet had an exit Re_h = 36,000, based on the slot height, h. The synthetic jets were operated in a periodic manner with a periodic momentum coefficient of about 3.3% and at a frequency of the main jet preferred mode. A short, wide angle diffuser of half angle of about 45° was attached to the main jet. Generally for the vectored jet, much of the flow features found here resembled those reported in the literature except that the deflection angle in this study increased with downstream distances inside the diffuser and then remained roughly unchanged thereafter. Larger jet spreading was achieved when the main jet was subjected to simultaneous actuation of the synthetic jets but the flow did not achieve the initial jet spreading that was observed in the vectored jet. Further jet spreading was achieved when the synthetic jets were alternately actuated in which each synthetic jet was actuated for a number of cycles before switching. This technique allowed the jet to flap across the flow between transverse positions larger than what would be obtained in a simple flip-flop jet. Under the present flow geometry and Reynolds number, it was found that when the ratio f_s/f_al, where f_s is the synthetic actuation frequency and f_al is the alternating frequency, was larger than 10, the mean streamwise velocity of the main jet had two peaks symmetrically placed with respect to the jet axis and the jet had the appearance of flowing into two streams each moving nearly parallel to the diffuser wall. For a value of f_s/f_al of about 10, the optimal value in this study, the desired flow properties were achieved in that, the mean velocity was nearly uniform with an increase in the jet width compared to the simultaneous actuations, and the jet flapping was more effective in redistributing and homogenizing the turbulent kinetic energy across the main jet.     

Separation control using synthetic vortex generator jets in axial compressor cascade

An experimental investigation conducted in a high-speed plane cascade wind tunnel demonstrates that unsteady flow control by using synthetic (zero mass flux) vortex generator jets can effectively improve the aerodynamic performances and reduce (or eliminate) flow separation in axial compressor cascade. The Mach number of the incoming flow is up to 0.7 and most tested cases are at Ma = 0.3. The incidence is 10° at which the boundary layer is separated from 70% of the chord length. The roles of excitation frequency, amplitude, location and pitch angle are investigated. Preliminary results show that the excitation amplitude plays a very important role, the optimal excitation location is just upstream of the separation point, and the optimal pitch angle is 35°. The maximum relative reduction of loss coefficient is 22.8%.The project supported by the National Natural Science Foundation of China (10477002 and 50476003) and the Ph.D. Innovative Foundation of Beihang University. The English text was polished by Yunming Chen.     

Active manipulation of a particle-laden jet

The interaction of a particle-laden jet (_ReUe=6600)$({\mathit{Re}}_{U_{\text{e}}}=6600)$ with a single synthetic jet or a continuous control jet located upstream of the main jet exit (i.e., within the main jet nozzle) was examined experimentally using PIV and PTV. A reduction technique was used to calculate 3-D flow fields from multiple 2-D measurement planes to study the complex 3-D interactions. The synthetic jet was shown to influence the particles both directly and indirectly through the manipulation of the carrier fluid’s drag force on the particles. The synthetic jet impulse directly vectors the particles away from the synthetic jet, while the formation of large vortical structures indirectly affects the particles, spreading throughout the measurement domain. By comparison, a continuous control jet only vectors the particles away from itself. The lowest Stokes number particles respond similarly to the carrier fluid, while higher Stokes number particles are less responsive to the control and only follow the strong vortical structures (i.e., higher circulation), which suggests that the preferential concentration concept depends on both the Stokes number as well as the strength of the coherent structures.     

压电激励圆形曲梁的静态响应及位移控制

研究压电激励圆形曲梁的静态位移响应及位移控制的参数特性。将压电夹层圆形曲梁等效为单层结构,基于一维小曲率曲梁理论,建立其控制方程。在集中弯矩和径向集中力以及电载荷作用下,分析了带压电激励器的圆形悬臂曲梁的静态响应。与有限元结果比较表明:本文的理论模型能够模拟压电激励的小曲率圆形曲梁的静态响应。压电夹层圆形曲梁在任意位置的径向集中力载荷作用下,控制其自由端径向位移响应为零,求得控制电压的解析表达,数值分析表明:随着集中力载荷的位置变化和梁长的变化,最优控制电压将出现峰值和反号。     

Using Stability Theory to Analyse Configurations of Pulsed Jets for Flow Control

During the last decade, efforts for simulating active flow control behavior by the use of pulsating jets have multiplied. In the present work, a URANS is used mostly for simulation, where the resulting flow characteristics can be reproduced with fair but adequate accuracy for engineering applications. This computational tool provides information concerning the effect on the flow, of the flow control. An additional computational tool is introduced, that of flow stability analysis, which allows to optimize the frequency and the position of the actuators (here pulsating jets). This tool will be developed through flow stability arguments. Both tools will be used within the context of the present paper and for reasons explained below, for suppressing only flow separation in internal flow cases. Once the computational tools are described/developed, they will be applied to a specific case. The optimization procedure will be demonstrated and discussed.     

Control of a submerged jet in a thin rectangular cavity

An innovative method is presented for control of an oscillatory turbulent jet in a thin rectangular cavity with a thickness to width ratio of 0.16. Jet flow control is achieved by mass injection of a secondary jet into the region above the submerged primary jet nozzle exit and perpendicular to the primary nozzle axis. An experimental model, a 2-D and a 3-D computational fluid dynamics (CFD) model are used to investigate the flow characteristics under various secondary injection mass flow rates and injection positions. Two-dimensional laser Doppler anemometry (LDA) measurements are compared with results from the CFD models, which incorporate a standard k–ε turbulence model or a 2-D and 3-D realisable k–ε model. Experimental results show deflection angles up to 23.3° for 24.6% of relative secondary mass flow are possible. The key to high jet control sensitivity is found to be lateral jet momentum with the optimum injection position at 12% of cavity width (31.6% of the primary nozzle length) above the primary nozzle exit. CFD results also show that a standard k–ε turbulence closure with nonequilibrium wall functions provides the best predictions of the flow.     

振动控制作动器的数目和位置优化设计

提出一种确定作动器的数目和优化设计作动器位置的方法。以独立模态最优控制方法为基础,将模态控制力和作动器作动力处理为随机变量,建立了模态控制力能量的自相关矩阵和作动器作动力能量的自相关矩阵。进一步通过作动力能量的自相关阵包含的能量确定了作动器的数目,在此基础上,建立了基于控制系统作动力消耗能量最小,优化设计控制系统的作动器最优位置。通过数值算例证明了该方法的有效性。     

Wing Buffeting Control at Transonic Flight Velocities with the Use of Ejector-Type Pulse Thermal Actuators

A new type of ducted pulse thermal actuators with a high pulse repetition frequency is proposed to control wing buffeting at transonic flight velocities. Ducted pulse thermal actuators can operate up to frequencies of about 1 kHz, which is sufficient for controlling the majority of aerodynamic processes at high subsonic flow velocities. As the use of a pulse thermal actuator in the regime of tangential injection of a jet is less efficient from the energy viewpoint than in the regime of boundary layer suction, an ejector-type pulse thermal actuator is proposed for implementation of the suction regime.     

Fluidic control of wake-flow behind a two-dimensional square back bluff body

This experimental study deals with wake-flow fluidic control behind a two-dimensional square back geometry positioned close to the ground. The fluidic control system is made of pulsed jets positioned at the upper edge of the model base. The objective of the fluidic action is to modify the wake-flow development, and as a consequence the static pressure distribution over the model base and hence the pressure drag. The main concern of this study is to determine to what extent the presence of a flow confined between the model and the floor influences the effectiveness of the control. Static pressure measurements at the model base and wake-flow characteristics derived from PIV measurements at a high acquisition frequency indicate global similarities between a case where an underbody flow exists and a case where this underbody flow is absent. For low actuation frequencies, discrepancies in the way the coherent structures due to the control develop in the shear layer appear.     

Jet interaction at supersonic cross flow conditions

The thrust produced by lateral jet systems has been successfully used for several years to control the flight trajectory, i.e., the maneuverability of spacecraft in the high atmosphere and in orbit. Recently this technology has also been applied to projectiles and rockets flying in the low atmosphere from sea level up to more than 10 km. At ISL, investigations have been performed with a 90 mm caliber full-scale projectile in order to study a special side jet controlling system at flight speeds of about 1500 m/s, i.e., Mach number at altitudes of 1.5 and 7.5 km. The High Energy ISL Shock Tunnel facility is used as a ground testing facility in which the flow around the projectile is studied at fully duplicated flight conditions. In the test facility the projectile is fixed inside the test chamber and the atmospheric air is set in motion flowing around the projectile test model. The air flow is generated in the ISL Shock Tunnel STB which is equipped for this purpose with a divergent square nozzle with an exit side length of 184 mm. A lateral gas jet is produced by combusting a solid propellant in a combustion chamber, placed inside the projectile. The powder gases are blown out laterally via a nozzle, creating a complex flow field by the interaction of the lateral jet with the external cross flow. Differential interferometry is used to visualize the behavior of the external flow field distorted by the lateral jet outflow. Numerical simulations have been performed based on steady state computations using the conservation equations of mass, momentum and energy. This was done to theoretically predict the development of the flow field around the projectile under the influence of the side jet. As final result the lateral force acting on the projectile is given as force and moment amplification factors, K_F and K_M respectively.Received: 7 May 2002, Accepted: 12 March 2003, Published online: 16 May 2003An abridged version of this paper was presented at the 23rd Int. Symposium on Shock Waves at Fort Worth, Texas, from July 22 to 27, 2001     

基于主动流动控制的射流矢量偏转技术

本文介绍了一种基于主动流动控制技术的射流矢量偏转新方法和控制思路。通过在主射流出口两侧加装斜置扩张固壁板来降低射流两侧与固壁边界间的流体压力,将射流偏转由"不敏感-难控"转变成"敏感-易控",再在固壁板布置自行研制的斜出口合成射流激励器对主射流进行比例偏转控制。实验结果表明,射流最大偏转角可达15°。此外还研究了激励位置角度、激励频率、激励电压不同工作参数对射流矢量偏转控制的影响,实现了主射流偏转角的比例控制。当合成射流与主射流动量比为1∶43时,主射流偏转角可达13°,合成射流激励器消耗的能量为1.5W,初步实现了以小的能量消耗获取高的控制效益。     

Application of flow suction for controlling the shedding of large-scale vortices at boundary-layer separation

A wind-tunnel study of the influence of flow suction on laminar boundary-layer separation behind a two-dimensional step on the surface is performed. Hot-wire measurements are carried out at low subsonic flow velocities. It is demonstrated that this method of flow control allows suppressing the formation of large-scale vortices determined by global stability properties of the separation region. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 4, pp. 60–65, July–August, 2006.     

Design and operation of the free jet elongational rheometer

The controlled free jet instability is used for elongational rheometry of polymer solutions. The instantaneous elongational viscosity represents a convenient comparative parameter. Its dependence on the operating parameters and the conditions of operation will be discussed.     

Numerical prediction of turbulent flow structure generated by a synthetic cross‐jet into a turbulent boundary layer

A detailed numerical study using large‐eddy simulation (LES) and unsteady Reynolds‐averaged Navier–Stokes (URANS) was undertaken to investigate physical processes that are engendered in the injection of a circular synthetic (zero‐net mass flux) jet in a zero pressure gradient turbulent boundary layer. A complementary study was carried out and was verified by comparisons with the available experimental data that were obtained at corresponding conditions with the aim of achieving an improved understanding of fluid dynamics of the studied processes. The computations were conducted by OpenFOAM C++, and the physical realism of the incoming turbulent boundary layer was secured by employing random field generation algorithm. The cavity was computed with a sinusoidal transpiration boundary condition on its floor. The results from URANS computation and LES were compared and described qualitatively and quantitatively. There is a particular interest for acquiring the turbulent structures from the present numerical data. The numerical methods can capture vortical structures including a hairpin (primary) vortex and secondary structures. However, the present computations confirmed that URANS and LES are capable of predicting current flow field with a more detailed structure presented by LES data as expected. Copyright © 2011 John Wiley & Sons, Ltd.